One problem often encountered in the production of natural gas from underground reservoirs is nitrogen contamination. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. Natural gases which contain a significant amount of nitrogen may not be saleable, since they do not meet minimum heating value specifications and/or exceed maximum inert content requirements. As a result, the feed gas will generally undergo processing, wherein heavier components such as natural gas liquids are initially removed, and then the remaining stream containing primarily nitrogen and methane is separated cryogenically. A common process for separation of nitrogen from natural gas employs a double column distillation cycle, similar to that used for fractionation of air into nitrogen and oxygen.
A recent significant advancement in such a process is described in U.S. Pat. No. 4,878,932--Pahade et al wherein the nitrogen-methane feed is separated using a single column nitrogen rejection unit (NRU) which also includes a phase separator. Another recent significant advancement in this field is disclosed in U.S. Pat. No. 4,664,686--Pahade et al wherein a stripping column is employed upstream of the NRU. These advancements enable the use of lower pressure feed for the separation.
It is desirable to recover residue methane at as high a pressure as possible in order to reduce pipeline compression requirements. One way of achieving this is to employ the compressed feed gas as a refrigeration source by means of Joule-Thompson or valve expansion of return streams. However, in low feed pressure situations the requisite feed compression is inefficient because the Joule-Thompson effect generated by returning nitrogen is small.
Accordingly, it is an object of this invention to provide a method wherein lower pressure nitrogen-methane feed may be more effecitvely employed in a nitrogen rejection unit.